The invention relates to a color display tube having three electron guns positioned in one plane for projecting three electron beams towards a display screen, and having a deflection device which is mounted on the display tube, said deflection device having a substantially frusto-conical supporting member with a front end facing said display screen having a large diameter and a rear end facing said electron guns having a relatively small diameter, which member supports a first set of deflection coils arranged about the longitudinal axis of said tube for deflecting the three electron beams in a first direction, and a second set of deflection coils arranged about said axis for deflecting the three electron beams in a second direction orthogonal to said first direction, the first set of deflection coils comprising two coils one of which is provided opposite to and spaced from the other in the supporting member with each coil being formed by two longitudinal parts extending substantially parallel to the said axis and being connected at their respective front and rear ends by transverse parts.
Due to mechanical spreads in the production and/or assembly of deflection coils with two longitudinal parts which are connected by transverse parts (so-called saddle-type coils) for deflection devices in color television sets, asymmetries can hardly be avoided.
Asymmetries in the coil halves cause certain picture defects. It is known from Netherlands Patent Specification No. 148,881 that such defects can be corrected by providing four auxiliary coils with the current through the turns of said auxiliary coils being in such a direction and of a value that a correction quadrupole field, which can be reversed in polarity, is generated which is substantially proportional to the deflection current through one deflection coil and/or to the deflection current through the other deflection coil.
This known (electromagnetic) correction device, which requires winding four extra coils and a circuit arrangement for energizing said coils, is complicated and hence expensive.